1. Field of the Invention
The present invention relates to a ventilation assembly using a variable speed motor which economically and reliably prevents fan motor operation below the minimum speed required for removal of excessive moisture and harmful pollutants from a ventilated building, and for adequate motor cooling and lubrication. More particularly, the invention relates to a ventilation assembly in which a temperature responsive speed controller automatically varies the speed of the motor in accordance with the building temperature and in which motor includes a centrifugal switch for activating an override relay in order to bypass the speed control to boost the motor speed to maximum for a predetermined time when motor speed drops below a predetermined minimum speed.
2. Description of the Prior Art
In typical livestock confinement buildings such as chicken houses, hog barns, and the like, temperature is controlled by means of a plurality of temperature responsive exhaust ventilation assemblies mounted to respective screened and louvered windows. Each ventilation assembly typically includes a motor coupled to a fan and a temperature responsive device of some sort such as a thermostat or temperature probe.
If thermostat control is used, a temperature rise above the set point causes the thermostat contacts close in order to energize the fan motor. This in turn exhausts air from the building which induces the entry of fresh air through intake vents in order to lower the temperature of the building. When the building temperature falls below the set point, the thermostat stops the motor. On and off operation of a motor typically results in a wider temperature band width than desired. Furthermore, on/off operation of the fan produces surges in air flow with potentially wide temperature swings especially if the outside air is substantially cooler than the building material. This can be particularly detrimental to temperature sensitive animals such as chickens. It can also result in higher energy costs due to increased cycling of heaters.
More sophisticated ventilation assemblies include a variable speed motor operably coupled with a speed controller which in turn is coupled with a temperature probe. In operation, the controller varies the speed of the motor in accordance with the temperature sensed by the probe. This allows a tighter control of temperature and results in a narrower temperature band width.
A typical prior art speed control varies the motor speed by varying the input voltage to the motor. In order to prevent the motor from running at a speed below the minimum required for adequate motor cooling and bearing lubrication, and for removing building moisture and pollutants, such speed controllers are designed to provide a minimum voltage level, such as 50%, to prevent the motor from operating below the minimum allowed speed, such as 20%. A problem develops, however, if the fan assembly is operating against a head wind or the louvers and screens become blocked with dust and dirt. When such occurs, the motor speed may drop below the minimum level even though the controller is providing sufficient voltage to operate the motor above the minimum level under normal conditions. This in turn may lead to motor burn out or, at least, shorten its expected operating life. It may also lead to serious under-ventilation of the building.